3 ms·

Another point, not mentioned in the article, is that you can't approach an MRI unit, unless you want to see the magnet rip out of your finger and ballistically

by pygy_ 14y ago

Another point, not mentioned in the article, is that you can't approach an MRI unit, unless you want to see the magnet rip out of your finger and ballistically destroy stuff on its way to the 1.5+ Tesla superconductive monster bathing in liquid helium. /dramatic exaggeration. Seriously, it can be very problematic, though. See http://www.simplyphysics.com/flying_objects.html http://www.simplyphysics.com/flying_objects.html

iamdann 14y ago
Yup, just like people with cheap "prison" tattoos.
- Nogwater 14y ago
  Not according to Mythbusters: http://dsc.discovery.com/tv-shows/mythbusters/mythbusters-database/mri-machines-and-tattoos.htm http://dsc.discovery.com/tv-shows/mythbusters/mythbusters-da...
  - iamdann 14y ago
    Additionally, people with magnet implants have successfully had MRIs, too. http://stevehaworth.com/main/?page_id=871 http://stevehaworth.com/main/?page_id=871
rtkwe 14y ago
Didn't know they were always or mostly energized. I thought they were only powered during an actual MRI scan.
- fossuser 14y ago
  They're super conductors so shutting them off involves draining all the liquid helium and nitrogen which is very expensive.
  - sp332 14y ago
    Well you can have a superconductor with no current running through it. Just charge the electromagnet with current during the scan, then bleed it off gradually when you're done. No need to turn off the superconductivity to power down the magnetism.
    - refurb 14y ago
      MRI magnets are a pain to setup. It's not an easy task. I can remember when the NMR (superconducting magnet as well) in our school "quenched". It's a hell of a lot of current to dissipate. Need to get some tech from Germany to come out and power it back up.
      - lostlogin 14y ago
        And $75k ish of helium - which is getting pricier by the day, and can at times be impossible to source. And the riskiest time in terms of quenches, is during ramping. Our last quench re-occurred just as they finished ramping it. We were down for 3 weeks ish and the cost excluding helium was huge.
- lostlogin 14y ago
  Ramping takes days (maybe even over a week). The RF is obviously off most the time though, and the gradient magnets are off too - these tend to be the evil bastards. The slew rate messes with your balance and after a few you come to dislike certain sequences - gradient echoes, diffusion and a few other - loud with high slew rates. You feel like your spinning and this causes slight nausea (in me) and 10 minutes or so of vertigo. Edit for spelling
skue 14y ago
> ballistically destroy stuff Not just "stuff," people have been killed this way. http://www.mrisafety.com/safety_article.asp?subject=39
- iamdann 14y ago
  People with magnet implants have had MRIs, though. http://stevehaworth.com/main/?page_id=871 http://stevehaworth.com/main/?page_id=871
  - lostlogin 14y ago
    Just hope that the area to be scanned isn't near the implant. The susceptibility artifact will ruin the images and mask nearby problems.
lostlogin 14y ago
MR tech here. I once had a minor accident with an angle grinder (well, I've had a few, but this one had side effects) and got several minuscule steel fragments in a finger. I didn't know this had happened, except that once at work (edit: I work on 3T scanners) I could feel them in my finger. They were slightly sore, and very annoying. A few x-rays later I managed to localise them and with the help of a colleague, dug them out with a needle. I never did find them, but we got them out. I have scanned quite a few people with various sorts of shrapnel in them. Only once was it a problem for the patient (moderate pain) but we don't take the scary ones in - intraocular metallic fragments, one deep in a neck etc.

by pygy_ 14y ago

iamdann 14y ago

Yup, just like people with cheap "prison" tattoos.

Nogwater 14y ago
Not according to Mythbusters: http://dsc.discovery.com/tv-shows/mythbusters/mythbusters-database/mri-machines-and-tattoos.htm http://dsc.discovery.com/tv-shows/mythbusters/mythbusters-da...
- iamdann 14y ago
  Additionally, people with magnet implants have successfully had MRIs, too. http://stevehaworth.com/main/?page_id=871 http://stevehaworth.com/main/?page_id=871

rtkwe 14y ago

Didn't know they were always or mostly energized. I thought they were only powered during an actual MRI scan.

fossuser 14y ago
They're super conductors so shutting them off involves draining all the liquid helium and nitrogen which is very expensive.
- sp332 14y ago
  Well you can have a superconductor with no current running through it. Just charge the electromagnet with current during the scan, then bleed it off gradually when you're done. No need to turn off the superconductivity to power down the magnetism.
  - refurb 14y ago
    MRI magnets are a pain to setup. It's not an easy task. I can remember when the NMR (superconducting magnet as well) in our school "quenched". It's a hell of a lot of current to dissipate. Need to get some tech from Germany to come out and power it back up.
    - lostlogin 14y ago
      And $75k ish of helium - which is getting pricier by the day, and can at times be impossible to source. And the riskiest time in terms of quenches, is during ramping. Our last quench re-occurred just as they finished ramping it. We were down for 3 weeks ish and the cost excluding helium was huge.
lostlogin 14y ago
Ramping takes days (maybe even over a week). The RF is obviously off most the time though, and the gradient magnets are off too - these tend to be the evil bastards. The slew rate messes with your balance and after a few you come to dislike certain sequences - gradient echoes, diffusion and a few other - loud with high slew rates. You feel like your spinning and this causes slight nausea (in me) and 10 minutes or so of vertigo. Edit for spelling

skue 14y ago

> ballistically destroy stuff Not just "stuff," people have been killed this way. http://www.mrisafety.com/safety_article.asp?subject=39

iamdann 14y ago
People with magnet implants have had MRIs, though. http://stevehaworth.com/main/?page_id=871 http://stevehaworth.com/main/?page_id=871
- lostlogin 14y ago
  Just hope that the area to be scanned isn't near the implant. The susceptibility artifact will ruin the images and mask nearby problems.

lostlogin 14y ago

MR tech here. I once had a minor accident with an angle grinder (well, I've had a few, but this one had side effects) and got several minuscule steel fragments in a finger. I didn't know this had happened, except that once at work (edit: I work on 3T scanners) I could feel them in my finger. They were slightly sore, and very annoying. A few x-rays later I managed to localise them and with the help of a colleague, dug them out with a needle. I never did find them, but we got them out. I have scanned quite a few people with various sorts of shrapnel in them. Only once was it a problem for the patient (moderate pain) but we don't take the scary ones in - intraocular metallic fragments, one deep in a neck etc.